White pigment on roof surfaces has been shown to generate interior temperatures that are as low as fourteen degrees (Fahrenheit) below temperatures sampled in buildings with dark roofs; it also has been shown to lower roof temperatures by up to seventy degrees (Fahrenheit), thereby extending the surface's useful life. Such white roofs are called “cool roofs”. While having a cool roof is beneficial to buildings during warm weather, it has obvious repercussions during cooler weather.
In a 1998 study, scientists at Lawrence Berkeley Laboratory in Berkeley, Calif., concluded that residents and businesses in the New York City area could cut their summertime air-conditioning costs by $22 million by swapping dark roofs with more reflective ones. Outdoor summer temperatures could also drop a couple of degrees. See, e.g., “Cooling Energy Savings Potential of Light-Colored Roofs for Residential and Commercial Buildings in 11 U.S. Metropolitan Areas”, S. Konopacki, H. Akbari, M. Pommerantz, S. Gabersek, and L. Gartland, Laurence Berkeley Laboratory, Environmental Energy Technologies Division Report, May. 1997; “Simulated Impact of Roof Surface Solar Absorptance, Attic, and Duct Insulation on Cooling and Heating Energy Use in Single-Family New Residential Buildings”, S. Konopacki and H. Akbari, Laurence Berkeley Laboratory, Environmental Energy Technologies Division Report, October 1998. On the other hand, the reflective roofs would raise heating bills by $6 million in winter. Hot roofs are preferable in wintertime. See “Scientists Watch Cities Make Their Own Weather”, Kenneth Chang, New York Times, Aug. 15th, 2000. As an example, FIG. 1 depicts an infra-red (IR) photo of a roof, of which the top part is coated and the bottom part is not. The coloration of the photo indicate that white coating reduced the roof temperature by about 40° C. (70° F.). See “Projects of the Heat Island Group: Demonstration of Energy Savings of Cool Roofs”, http://eetd.lbl.gov/HeatIsland/PROJECTS/DEMO/.
While the notion of enjoying the benefits of a white roof (or “cool roof”) has been accepted and applied for many years, there is no technology in the market that allows buildings to capture the value of cool roofs during warm weather without losing value during colder weather. Thus, within the American market, cool roofs are widely used throughout regions that do not experience dramatic variances in temperature, such as the extreme southern regions of the United States. However, most areas of the country cannot afford to take advantage of cool roofs because of the consequences that would be incurred during colder months. Consequently, there is a need for a roofing system that can change its energy reflectance based on the outdoor temperature. For example, FIG. 2 depicts a graph showing the potential net energy savings from changing roof reflectivity. Savings are measured in dollars. Note that the net savings are the savings of cooling energy use less the penalties of heating energy use.